2 Answers

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Best answer

Here is what you are looking for.

Uxie:

Method:
After completing the game, if you visit the Cave of Being, located within Route 20, Professor Juniper will see the three pixies shooting off around Unova and ask you to follow them. The first, Uxie, can be found outside the Nacrene City Museum. You need to stand on a particular spot in order for it to appear -Serebii

Azelf:

Method:
After completing the game, if you visit the Cave of Being, located within Route 20, Professor Juniper will see the three pixies shooting off around Unova and ask you to follow them. The final one, Azelf, can be found well in the left hand side of Route 23. You need to stand on a particular spot in order for it to appear -Serebii

Mesprit:

Method:
After completing the game, if you visit the Cave of Being, located within Route 20, Professor Juniper will see the three pixies shooting off around Unova and ask you to follow them. The second, Mesprit, can be found on the summit of the Celestial Tower. You need to stand on a particular spot in order for it to appear -Serebii

Mesprit is being of emotion, = Ceslestic tower, where pokemon passed away.
Uxie is being of knowledge, = Nacrene Musem, where there is knowledge.
That's cool. But Azelf is will power, and i'm too lazy too figure out what is special about rt. 23